Polygyny has far more historical precedent than "gay marriage". I see no particular reason why "gay marriage" should be acceptable but polygyny not.
The most important function of marriage is ensuring that children will have a known mother and father. Polygyny accomplishes this; polyandry and gay marriage do not.
Wrong. The courts have ruled that women have the right to deceive men as to the paternity of their children. Thus, the most important function of marriage in our society is to transfer money to women. It's merely a way to subsidize the welfare state--an excise tax on men.